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High speed miss -UPDATE- we have a winner.

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Heviduty View Drop Down
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    Posted: 08 Jan. 2022 at 8:08pm
47 L134  purchased this as a basket case.    Got it running and now it runs very well… except.  At about 55 MPH in overdrive  it feels like a cylinder drops out.    Not random but more of a dead hole.  Drop back to fifty and it takes ten seconds , stumbles a bit and picks up the fourth again. After it picks up again I can stomp on the gas and it pulls strong again up until it drops a cylinder again.  In direct drive the same thing happens but about ten miles per hour slower.  
Adjusted the valves. Compression at 105 PSI and even. Good fuel pressure. Fuel cap is vented.   New points with strong spring so don’t suspect points floating. Dwell set at 42. New advance springs installed and adjusted. New cap and copper core wires.  Plugs have good color.  The bushings in the distributor feel ok, maybe not perfect. 
  I did replace the condenser but not the coil. 
  Could a coil  cause this.  I have a new one on hand but ran out of time today. Will switch it out tomorrow.  What have I overlooked? 

 Update , I raised the metering rod a bit and now she runs great.    That’s not to say the high speed miss has entirely gone away but it’s only a factor above sixty MPH.  Got her up to 65 and I can safely say “that ain’t ever gonna happen again” 
   Between bald tires and questionable alignment I may or may not have soiled myself.      
   


Edited by Heviduty - 14 Jan. 2022 at 6:58pm
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Oldpappy View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Oldpappy Quote  Post ReplyReply Direct Link To This Post Posted: 08 Jan. 2022 at 8:26pm
The coil is not likely to be the culprit.

Perhaps you should re-check the mechanical advance in the distributor. These are often overlooked, and not usually well maintained. You said new springs, check the pivot pins are not worn out on the weights.

 


Edited by Oldpappy - 08 Jan. 2022 at 8:27pm
If you can't get there in a Jeep you don't need to be there
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Heviduty Quote  Post ReplyReply Direct Link To This Post Posted: 08 Jan. 2022 at 9:06pm
Oldpappy; The springs were gone when I first opened it up.     I put in new springs and did bend the mount pins out slightly as there was quite a bit of slop. Everything is free and well lubed now. What confuses me is that the miss isn’t random but only one cylinder.    Never more and never less.  Also there is no vacuum wipers so no lines to leak causing a lean on #4 cylinder.  
   I did check for intake leaks early during my initial startup. 
  I’m thinking a single cylinder is breaking down but I’ve changed a bunch of stuff with no effect on this. 
I’m not thinking a coil issue either but that’s all that I haven’t either confirmed good or replaced. 
  I might push it hard and let it miss for a minute, kill the ignition and coast to a stop.  Might show a wet plug.   Won’t fix anything but if I can identify which hole goes dead then I can shuffle things and try to move the issue to another cylinder.  Then I can learn things.  
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Bruce W Quote  Post ReplyReply Direct Link To This Post Posted: 08 Jan. 2022 at 11:25pm
  This could be a worn distributor. The main shaft bushings and shaft wear, and also the breaker cam assembly wears itself and the top of the main shaft where it pivots. Remove your distributor cap, rotor and dust cover, turn the engine over until the points rubbing block is on a peak of the cam, then push the cam back and forth towards/away from the points and see how much movement there is. I don’t know what the limit is, but I think if you can see movement it’s too much. 
  Do you have carbon-core or copper-core plug wires? Six or twelve volts?
BW 
It is NOT a Jeep Willys! It is a Willys jeep.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote athawk11 Quote  Post ReplyReply Direct Link To This Post Posted: 08 Jan. 2022 at 11:30pm
I had a similar experience when driving fast or climbing a hill. The glass cover on my fuel filter was loose enough to allow air to be sucked in, but only when I was really riding the accelerator.  Once I let off the accelerator and coasted for a bit, the air leak would reduce and allow the carburetor to fill back up with fuel. The stumble would go away...until the next hill or high-speed situation.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Heviduty Quote  Post ReplyReply Direct Link To This Post Posted: 08 Jan. 2022 at 11:47pm
Bruce W;    I checked that earlier and there is some slight movement.   I’m not sure how much is a problem.   It seems very slight to me but…. It’s something. I also have a very slight stumble at idle and understand this also could be a result of worn distributor bushings.      There was quite a bit of wear on the advance spring anchors so I can imagine there may be high hours on this distributor. The distributor is an   IGC 4705 which is from an early GPW if I understand correctly. I guess I could rebush this one but….. 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Boomer Quote  Post ReplyReply Direct Link To This Post Posted: 08 Jan. 2022 at 11:49pm
you may have a weak spark plug. Maybe one that at higher revs is not making the right amount of spark and then at slower speed it takes it a minute or so to clean out and start working

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Heviduty Quote  Post ReplyReply Direct Link To This Post Posted: 08 Jan. 2022 at 11:58pm
athhawk11; 
This one I can rule out.   Only because I was convinced that this was the whole problem for quite some time.    The mech. fuel pump looked new but was bypassed and an electric one was in place when I took over this project. It had failed and I replaced it with a cheap Mr Gasket pump from Napa. (Only one available at the time).       It was in fact starving for fuel until I tossed Mr Gasket and the pressure regulator and installed an electric pump under the tank.   I now hold no less than three pounds fuel pressure at all times, even during the stumbling episodes. 
     Oddly enough when I removed the mech. pump to pull the valve cover the pump arm was missing, gone.    
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Heviduty Quote  Post ReplyReply Direct Link To This Post Posted: 09 Jan. 2022 at 12:01am
How hard is it to find bushings to rebuild this less common dizzy I’m working with?      Perhaps I should send it out but that goes against my personal philosophy. 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Heviduty Quote  Post ReplyReply Direct Link To This Post Posted: 09 Jan. 2022 at 12:07am
Boomer;  yup, my intent is to carry an entire set of all tune up parts. I will get another set of plugs and switch them out.   These will get cleaned ,  vacuum packed and tossed in the toolbox.   
   Maybe one was not gapped right or something.  Great advice , thanks. 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Heviduty Quote  Post ReplyReply Direct Link To This Post Posted: 09 Jan. 2022 at 12:36am
Bruce w :
  I removed carbon core wires and installed copper core.    No change.   
   This has been converted over to 12V. 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Heviduty Quote  Post ReplyReply Direct Link To This Post Posted: 09 Jan. 2022 at 12:42am
Honestly this thing runs good at 50 MPH.    Before I got the alignment from 1.5” toe out to about 1/4” toe in I couldn’t keep it on the road.      It’s a whole lot better now but 50 is fast enough.      Getting the bald tires off will help with the ditch dives.   
   I could live with 50 as a top speed but it isn’t right.  I like things as right as is reasonably possible.    


Edited by Heviduty - 09 Jan. 2022 at 2:12am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Michaeltru Quote  Post ReplyReply Direct Link To This Post Posted: 09 Jan. 2022 at 2:02am
Now if you could find a shop with a dyno and engine analyzer, you could diagnose it while the jeep is stationary. Or hopefully diagnose it. Learned that in a community college course in the ‘70s
Mike in AZ
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Rus Curtis Quote  Post ReplyReply Direct Link To This Post Posted: 09 Jan. 2022 at 10:18am
This is an interesting puzzle.  I'd initially wondered if a valve could be sticking.   

But I'd had multiple distributor issues early on and a replacement fixed that problem.  I could also imagine arcing in the cap at high speed, but I'm only guessing.

The suggestions so far are worth pursuing, including your idea of a quick shut down to inspect the spark plugs to hopefully help identify the cylinder. 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote TERRY Quote  Post ReplyReply Direct Link To This Post Posted: 09 Jan. 2022 at 10:59am
Is there a good spring on the points? If the spring is weak you could be getting point float or bounce at high rpms.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Heviduty Quote  Post ReplyReply Direct Link To This Post Posted: 09 Jan. 2022 at 12:32pm
TERRY;
 As to point bounce I would think if this was the issue it would go away immediately upon reducing RPM. It lingers for ten seconds or so at lower engine speed.     Perhaps this is the time required for the plug to clear and start firing properly again.   
  I guess that I could try a higher RPM run in maybe second gear and see if I can recreate this condition at lesser load.  That would rule out point float. I’m not sure I would notice a cylinder drop out without a load on the engine. 
  It pulls itself along pretty good on three cylinders. Can maintain about 50 MPH. 
    I have not tried another set of points yet. The replacements were the brand “standard” which I understand is a well respected brand on here.   During the installation of the points what did stand out was the spring tension.  It was noticeably  more than the points that came with UH-OH. We named the Jeep UH-OH. The little woman didn’t think calling it “ROLLING THUNDER” sounded appropriate.  She suggested “stumblesmoke” 
  I’ll grab another set of points and swap these out. I need a spare set for the tool box anyway. 
  Thanks , that gives me something else to double check.  
As to arcing in the cap, I replaced the cap. No joy. 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Heviduty Quote  Post ReplyReply Direct Link To This Post Posted: 09 Jan. 2022 at 12:46pm
RUS CURTIS; 
 I’m pretty sure it’s not a valve.   Early on it ran very poor.   Acted like a sticking valve.   I poured all manner of cocktails down her, sea foam , MMO , corrosionX , PB blaster etc. 
   Nothing seemed to help.  Resigned myself to busting stuff , removed the fender and adjusted the valves.   
  Nothing broke and I discovered two things.    Some valves were running with less than 3 thou. Clearance and perhaps the cleanest engine I’ve ever been into.  
     The previous owner drove UH-OH as a daily driver for years and when it broke it went into heated dry storage. A borescope shows very clean cylinders. Perhaps the results of a quart of sea foam down the  throat.   Perhaps an overdose of Marvel Mystery Oil in the fuel.   I see very little carbon and no corrosion to stick a valve.  
If I had access to a known good unit I’d shove a new dizzy into her in a second.   I may get up there in the end but I’ve not heard great things about new replacements.    I may stumble across a good used dizzy some day. 


Edited by Heviduty - 09 Jan. 2022 at 12:51pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Lee MN Quote  Post ReplyReply Direct Link To This Post Posted: 09 Jan. 2022 at 12:48pm
I would take a very close look at your valve springs, you may have a broken one or two ?…. Worth a look at this point.

Lee😉
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